Devices such as a cellular phone, a digital camera, a PDA, a touch panel display, a large-screen television, and wireless lighting show a tendency of further prevalence.
A tempered glass, which is produced by applying tempering treatment to glass through ion exchange treatment or the like, is used for those applications (see Patent Literature 1 and Non Patent Literature 1).
The tempered glass has been particularly used in recent years for a protective member for a display of a large-screen television. Such protective member is required to have, for example, the following properties: (1) having high mechanical strength; (2) having a liquidus viscosity suitable for a down-draw method such as an overflow down-draw method or a slit down-draw method, a float method, and the like, in order to form a large number of large glass sheets; (3) having a high temperature viscosity suitable for shape formation; and (4) being able to be produced by carrying out tempering treatment inexpensively and efficiently.